The acoustic radiation force: a gravitation-like field

In this letter, we propose an expression for the \emph{instantaneous} acoustic radiation force acting on a compressible sphere when it is immersed in a sound field with a wavelength much larger than the particle size (Rayleigh scattering regime). We show that the leading term of the radiation force can alternatively be expressed as the time average of a fluctuating gravitation-like force. In other words, the effect of the acoustic pressure gradient is to generate a local acceleration field encompassing the sphere, which gives rise to an apparent buoyancy force, making the object move in the incoming field. When averaging over time, we recover the celebrated Gor'kov expression and emphasize that two terms appear, one local and one convective, which identify to the well-known monopolar and dipolar contributions.